In a Mannesmann tube-making method, which has been widely used in a representative tube-making method for a seamless tube, a solid round billet heated at a predetermined temperature is processed by a piercer mill into a hollow tube stock. Then, said method is outlined that the obtained hollow tube stock is elongated by a mandrel mill consisting of 5 to 8 stands, reheated and processed, or alternatively being processed without reheating, by either a stretch-reducer mill or a sizer mill to obtain a predetermined outside diameter, subsequently followed by a finishing process, thereby obtaining a final product.
In a piercing-rolling process by a piercer mill, a pair of barrel-type or cone-type rolls whose center lines are inclined with respect to the pass line are disposed as opposed to each other. Further, a plug for use in piercing-rolling process is held at the tip of the mandrel, that is provided along the pass line lying between a pair of inclined rolls.
Normally, the cone-type inclined rolls are adopted as a piercing roll for use in piercing-rolling process since the quality of the pierced goods is excellent and the efficiency of piercing operation is high.
FIG. 1 is a diagram explaining schematically a configuration of cone-type inclined rolls for use in a piercing-rolling process. FIG. 2 is a diagram, seen in the direction of an arrow A—A for the foregoing diagram, explaining a configuration of cone-type inclined rolls.
The inclined rolls 1 comprises a gorge 1a having a roll diameter Dg in its mid-span, an inlet face 1b which is a side face of a near truncated circular cone in such a manner that the diameter decreases from the gorge 1a toward the inlet endmost, and an outlet face 1c which is a side face of a near truncated circular cone in such a manner that the diameter increases toward the outlet endmost, thereby resulting in the cone-type form as a whole.
The inclined rolls 1 are disposed so that each centerline of the rolls crosses the pass line X—X at a cross angle γ respectively. Further, as shown in FIG. 2, a first inclined roll 1 is disposed so that the center line of the roll crosses the pass line X—X at an inclination angle β. Meanwhile, another inclined roll 1 that is not shown in FIG. 2 is disposed as opposed to the first inclined roll so that the centerline of another roll crosses the pass line X—X at a reverse angle β which is symmetric with respect to the pass line X—X.
The inclined rolls 1 intended for applying a rotational movement to the billet 3 are directly connected with each driving mechanism 4, thus enabling each roll to independently rotate around its centerline.
And a plug 2 is configured to be an artillery shell form as a whole, and the rear end thereof is held at the tip of the mandrel bar M. Further, the rear end of the mandrel bar M, not shown in the diagram, is connected with the thrust block mechanism which can provide forward or backward movement in the length-wise direction to sustain the thrust force in rolling direction which being exerted onto the plug 2, as well as to adjust the position of the plug.
In a piercing mill with the foregoing configuration, when the billet 3 that is mobilized along the pass line X—X in the direction shown by a hollow arrow travels along the pass line X-X while rotating at the in-between space of the inclined rolls, a borehole is made by the plug to the centerline of the billet and subsequently a wall thinning by means of the inclined rolls 1 coupled with the plug 2 takes place, thereby obtaining a hollow tube stock.
Meanwhile, in the foregoing piercing-rolling process, the billet is subjected to a piercing-rolling process by a pair of the inclined rolls while rotating and moving forward during the period from being fed into the inclined rolls to the travel down to the nose of the plug. In this regard, so called rotary forging effect (Mannesmann effect) works, resulting in rendering the centerline portion of the billet fragile, thus reaching a state that a piercing-rolling process can be readily executed. In case the rotary forging effect is excessive, the voids likely generate at the centerline portion, and in extreme case the centerline portion gets fractured, likely resulting in radial cracking.
In such a case, especially when a continuously cast material likely having a center segregation and/or a center porosity, a stainless steel with 5% or more Cr likely having δ ferrite, or a non-ferrous billet such as copper or copper alloy that a dendrite structure likely remains and impairs the workability is subjected to a piercing-rolling process by a piercer mill, cracks develop at the centerline portion of the billet due to the rotary forging effect during the period from being fed into the rolls to the travel down to the nose of the plug, thus being left behind as internal surface flaws of a hollow tube stock after rolling operation. In order to eliminate these internal surface flaws, various methods are proposed up to date.
In general, in a piercing-rolling process by a piercer mill, a plug position and an opening angle between the inclined rolls are adjusted to set a smaller draft rate of the billet at the position where the nose of the plug is located (a plug nose draft rate). For instance, in Japanese Patent Application Publication No. 03-13222, after the billet is engaged with the inclined rolls, it is disclosed that an opening angle of the inclined rolls and the plug lead are altered simultaneously so that a plug nose draft rate in the state of rolling at the mid-length of the billet becomes smaller than the plug nose draft rate in the state of rolling at the top or bottom of the billet.
According to the rolling method disclosed in foregoing Japanese Patent Application Publication No. 03-13222, a miss-rolling such as defective onset engagement with the rolls can be prevented and the occurrence of internal surface flaws attributable to the excessive rotary forging effect, excluding the top, and bottom portion of the hollow tube stock, can be avoided. However, since the onset engagement with the rolls at the top of the billet is mostly cared, it is suspected that the occurrence of internal surface flaws at the top of the hollow tube stock cannot be completely prevented. Also, a development of a new equipment which can alter the setting of the inclined rolls during a piercing-rolling process will be required.
Next, in Japanese Patent Application Publication No. 61-3605, there is disclosed a piercing method, wherein, based on the billet weight and the target dimension of the hollow tube stock, an opening angle of the inclined rolls and a plug lead setting are controlled so as to get a targeted value of the plug nose draft rate, thus preventing the occurrence of internal surface flaws. According to the proposed controlling method, it is suspected that even if the opening angle of the inclined rolls and the plug lead are set to the targeted values in accordance with the variation of a steel grade of the work material, the shape of the inclined rolls, a condition for piercing-rolling process and the like, the occurrence of miss-rolling such as defective onset engagement with rolls can not be completely prevented, although the occurrence of the internal surface flaws of the hollow tube stock may be prevented.
Further, in Japanese Patent Application Publication No. 2000-140911, there is disclosed a method of a piercing-rolling process, wherein the inclination angle of the inclined rolls is specified to be 12°–14° and, at the same time, the piercer mill is operated so that the ratio of the distance—from the position where the billet initially get engaged with the rolls to the nose of the plug—to the billet diameter becomes a specific condition, thereby enabling to prevent the occurrence of the internal surface flaws.
In the method of a piercing-rolling process disclosed in foregoing Japanese Patent Application Publication No. 2000-140911, it is suspected that the internal surface flaws especially at the top of the hollow tube stock cannot be completely prevented, although the miss-rolling and the internal surface flaws may be marginally prevented by said invention, similarly to the piercing-rolling process disclosed in foregoing Japanese Patent Application Publication No. 03-13222.
Thus, in case of a piercing-rolling operation for aforementioned hard-working material such as a continuously cast material and a stainless steel with 5% or more Cr that likely generates δ ferrite, there is a risk that internal surface flaws numerously generate at the top of the hollow tube stock. Further, a development of a new equipment that enables altering the roll setting during a piercing-rolling operation is required.